Gas emitting device

ABSTRACT

A device for emitting a gas at a constant rate into a moving fluid medium to produce an accurately known concentration of the gas in the medium. The gas is held in a small cylinder under pressure in equilibrium with its liquid phase or solely in its gaseous phase, and permeates through a silicone material filling an accurately dimensioned passage through one end of the cylinder. Permeation rates may be selected from a wide range by varying the dimensions of the passage, and the device has a relatively low sensitivity to temperature variations.

United States Patent 11 1 I Chand Dec. 24, 1974 GAS EMITTING DEVICEPrimary Examiner-Lloyd L. King 75 I h 1 mentor Ramesh C and Santa SusanaCallf Attorney, Agent, or Fzrm-Fulw1der, Patton, Rleber, [73] Assignee:Ecology Board, Inc., Los Angeles, Lee & Utecht Calif. [22] Filed: Mar.12, 1973 ABSTRACT [21] Appl. No.2 340,300 A device for emitting a gas ata constant rate into a moving fluid medium to produce an accuratelyknown concentration of the gas in the medium. The gas is [52] US. Cl.239/34 held in a Small cylinder under pressure in equilibrium [51] Int.Cl. A24f 25/00, A611 9/04 with its liquid phase or l l i its gaseousphase, and [58] Fleld of Search 239/34, 58; 206/5 permeates through asilicone material filling an accu I rately dimensioned passage throughone end of the [56] References C'ted cylinder. Permeation rates may beselected from a UNITED STATES PATENTS wide range by varying thedimensions of the passage, 3,412,935 1 11/1968 OKeeffe 239/34 and thedevice has a relatively low Sensitivity to 3,655,129 4/1972 Seiner239/34 perature variations. 3,679,l33 7/1972 Sekiguchi et a. 239/343,788,545 1/1974 Budd et al. 239/34 29 Clams, 5 D'awmg GAS EMITTINGDEVICE BACKGROUND OF THE INVENTION This invention relates generally todevices for the emission of a gas at a constant rate, and, moreparticularly, to such devices used'in the production of calibrationsamples for gas or liquid analyzers, and in which the gas is emittedthrough a permeable material into a moving fluid medium.

Previously available devices of this type have employed an elongatedtube of a permeable, polymeric plastic material to hold a gas underpressure and partially in the liquid phase. At a constant temperature,the vapor pressure of the substance will be constant and molecules ofthe substance will permeate through the walls of-the tube at a constantrate, thereafter to be intermixed or dissolved in a constantly movingfluid medium stream surrounding the tube. Tube devices of this type aredescribed in a patent to OKeeffe entitled Gas Dispensing Devices, US.Pat. No. 3,412,935.

Although these permeable tube devices were a significant advance in thefield, they suffer from two major disadvantages. Firstly, the rate ofpermeation of the substance through the tube walls is highly sensitiveto temperature variations. An increase in permeation rate resulting froman increase in temperature can be considered as having two components:one due to a vapor pressure increase resulting from the temperatureincrease, and the other due to an inherent property of the permeablematerial that results in an increase in permeation rate withtemperature, at constant pressure. In

, the prior art tube device, these two components are cumulative, and anadverse temperature characteristic results. Accordingly, the device mustbe maintained at a constant temperature to a high degree of accuracy ifthe permeation rate is to be held constant, and this problem is furtheraggravated if the tube is relatively long. since the constanttemperature must be maintained over the entire length of the tube.

The second disadvantage of the prior art tube devices is that they aresuited only for the production of very low concentrations of the gaseoussubstance. In another version of the prior tube devices, the substancepermeates through a generally annular permeable element at one end ofthe tube, but this device produces even lower permeation rates, andconcentrations down to l millionth of a part per million. For someapplications, such as the calibration of ambient monitors for thedetection of pollutants in the atmosphere, only small concentrations areinvolved. For example, the

concentration of sulfer dioxide in the atmosphere rarely exceeds 1 partper million '(ppm), and the tube devices are well enough suited toproduce calibration samples with this order of concentration. For otherapplications, however, such as the calibration of stack monitors" forthe analysis of effluent gases from industrial chimney stacks or fromengine exhausts, higher concentrations are often encountered. Forexample, S0 concentrations are typically in the range 100 to 1,000 ppm,and it is difficult and highly inconvenient to obtain calibrationsamples in this range with the tube devices of the prior art.

Higher concentrations can, of course, be obtained by decreasing the flowrate of the fluidmedium used as a carrier, but this has practicallimitations and too low a flow rate reduces the concentration accuracyof the resulting mixture. A larger tube will also result in higherconcentrations, but again there are practical limitations, since alength or diameter increase of several hundred times may be required toobtain the desired range of permeation rates. As a practical matter,higher concentration calibration samples have hitherto been obtainableonly by premixing gases in very large cylinders, and this methodproduces an inherently unreliable sample, especially after some time haselapsed between preparation and use of the sample, as is usually thecase.

It is apparent, therefore, that there has been an urgent need for adevice for the preparation of calibration samples which retains theadvantages of prior devices, but which is less sensitive to temperaturevariations, and which may be made to produce a wide range ofconcentrations in the samples. The present invention fulfills this need.

SUMMARY OF INVENTION The present invention resides in a new device foremitting a gaseous substance at a predetermined rate into a moving fluidmedium. Briefly, and in general terms, the device includes a sealedvessel for holding the substance to be emitted under pressure, thevessel having a passage filled with a permeable polymeric plasticmaterial through which the substance permeates and is emitted outsidethe vessel.

The permeable polymeric plastic material used in the device has a muchhigher permeation rate and much less sensitivity to temperaturevariations than the material used in prior devices. This allows the useof a novel structure having a relatively low cross-sectional area ofpermeable material, but a relatively high rate of emis sion of thesubstance. Furthermore, the substance may be held either in equilibriumwith its liquid phase or entirely in its gaseous phase, the latterresulting in an even more favorable temperature characteristic for thedevice. Also, the rate of emission can be widely and easily variedbyemploying different dimensions for the passage, without increasing theoverall size of the device except as is required to increase the life ofthe device between refills.

More specifically, in a presently preferred embodiment of the invention,the vessel takes the form of a hollow metal cylinder, closed at one endexcept for the passage which is formed therethrough, and adapted at theother end to receive a pressure seal. The polymeric plastic, a siliconematerial in the presently preferred embodiment, is positioned in thepassage and cured to form a permanent bond with the metal, after whichthe vessel is evacuated and filled with a liquefied gaseous substance.In a variation of this preferred embodiment, the gaseous substance isstored in the cylinder under a pressure not sufficient to liquefy thesubstance. The substance may be any of a wide variety of fluids,including water, and the passage in the vessel can be dimensioned toprovide any emission rate over a wide range,

' to meet the requirements of various applications.

Furthermore, the structure of the present invention allows easyconnection with the moving fluid medium by means of a simple T-junction.Thus, the device may be readily adapted for use with any analyzer or maybe conveniently housed integrally with an analyzer.

It will be appreciated from the foregoing that the present inventionovercomes the major disadvantages of prior art devices, in thatsensitivity to temperature variations is considerably lessened andcalibration samples having a wide range of concentrations can be easilyproduced. Other aspects and advantages of the invention will becomeapparent from the following detailed description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS ture characteristic of the device ofFIG. 1 utilizing the gaseous and liquid phases of the substance; and

FIG. 5 is a graph illustrating the temperature characteristic of thedevice utilizing only the gaseous phase of the substance.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawings forpurposes of illustration, the device of the present invention is used todispense a gaseous substance at a constant rate, the gas thereafterbeing mixed with a moving fluid medium to provide a sample containing aknown concentration of the gas, which is then typically used tocalibrate an analyzer of gases or liquids. The device includes a sealedvessel (FIGS. 1-3) in which the gaseous substance is held underpressure, and a quantity of permeable polymeric plastic material 11through which the substance permeates and escapes from the vessel.

In accordance with the present invention, the vessel 10 is formed from amaterial impervious to the substance contained therein, and thepermeable plastic material 11 fills a passage 12, as best shown in FIG.2, extending from the inside to the outside of the sealed vessel. Thestructure uses a permeable material such as silicone, which has lesssensitivity to temperature variations than materials used in priordevices, and a higher permeation rate/Thus, the device has a moredesirable temperature characteristic than existing devices, and the rateof emission of the gaseous substance can be preselected from a widerange of rates by proper selection of the dimensions of the passage 12.

More specifically, the vessel 10 employed in a presently preferredembodiment of the invention is a hollow cylinder of stainless steel orother material impervious to, and practically non-reactive with thesubstance to be contained therein. As best illustrated in FIG. I, thecylinder 10 has an open upper end 13 threaded internally to receive apressure seal 18, which is normally not removed except for refilling thecylinder, and a solid lower end 14 of substantial length through whichthe passage 12 extends along the axis of the cylinder. The terms upperand lower as used herein are descriptive only with respect to thefigures, and the device is not limited to operation in a particularorientation.

The cylinder 10 has a reduced diameter, both inside and outside, over asubstantially long lower end portion 17 of the cylinder, to facilitatefitting the device to pipes containing the moving fluid into which thegaseous substance is to be emitted. Accordingly, the lower end portion17 has a threaded nut 19 and an associated sealing ring 21 fittedthereto for engaging a threaded pipe fitting 22.

For relatively low permeation rates, the cylinder 10 is typically 0.5inch in outside diameter, reducing to 0.25 inch in diameter at its lowerend 17 for convenient fitting to quarter-inch diameter gas lines. Thepassage 12 in this size of cylinder 10 is typically a few hundredths ofan inch in diameter and about a quarter-inch long. For higher permeationrates, the cylinder 10 may be 2 or more inches in diameter and 8 or 9inches long, to provide a reasonably long life between refills, and thepassage 12 may be a half-inch or more in diameter. The higher permeationrates may be obtained by increasing the diameter of the passage 12 ordecreasing its length, or both.

The permeable material 11 used in the presently preferred embodiment isa silicone polymeric compound chemically classified as dimethylpolysiloxane. Many compounds of this type are commercially available andcan be used in the present invention. By way OfICXflI'II- ple, thedimethyl compound PR1939, manufactured by Products Research and ChemicalCorp., Los Angeles, California, and dimethyl RTV compound 630,manufactured by General Electric Co., Waterford, New York, have eachbeen found suitable in the presently preferred embodiment.

Before the silicone material .11 is positioned in the passage 12, theentire cylinder 10 is cleaned with a degreasing agent to remove anypossible contaminants. Then the silicone material 11, which is typicallyrelatively viscous, is drawn into the passage 12 by applying a partialvacuum to the cylinder 10 at its upper end 13. When the siliconematerial 11 completely fills the passage and any trapped air bubbleshave been removed, the partial vacuum is removed and the siliconematerial is cured at a temperature of approximately F, preferably for atleast 24 hours. During curing, the silicone material 11 hardens andbecomes permanently bonded to the walls of the passage 12.

Two variant forms of the preferred embodiment are presentlycontemplated. In the first, the gaseous substance is introduced into thecylinder 10 in a liquefied state, and is held there under pressure withits liquid and gaseous phases in equilibrium, as was the case in theprior tube devices. In the second form, the substance is held at a lowerpressure entirely in its gaseous phase. Although this latter formsuffers from a shorter life between refills and a gradual fall inpermeation rate as the cylinder 10 is emptied of gas, it can still bedesigned as a practical device, and, as will be discussed in detailbelow, it has the advantage of a very favorable temperaturecharacteristic.

After the silicone material 11 has been cured in the cylinder passage12, the cylinder 10 is evacuated and filled with the desired gaseoussubstance through the open upper end 13. If the substance is normallygaseous at room temperature, and is to be introduced in the liquidstate, the cylinder 10 is usually cooled below the boiling point of thesubstance to minimize loss of liquid when the cylinder is sealed.

FIG. 3 diagrammatically illustrates how the device of the presentinvention would typically be connected for use. For calibrating ananalyzer 22, the cylinder 10 is connected by a T-junction 23 to emit thegaseous substance at a constant rate through the silicone 11 into thefluid medium flowing at a constant rate along a pipe 24 to the analyzer.The fluid medium may be an inert gas, such as nitrogen, in which theemitted substance is mixed, or, in some applications, it may be a liquidin which the emitted substance is dissolved. The fluid medium is drawnfrom a supply system 25, which typically includes a pump, a pressureregulator and a flow meter, none of which is illustrated.

A constant temperature is preferably maintained by means of some heatingelement 27 wrapped around the cylinder and supplied with power through acontrol unit 28 which includes an adjustable thermostat (notillustrated), thus maintaining a constant rate of emission of thesubstance contained in the cylinder 10.

1 When liquefied gas is used, it has been found that a constanttemperature need be maintained only to within approximately 0.5C,whereas a variation of 01C was all that could be tolerated in priordevices with comparable accuracy, In the form of the invention in whichthe substance is held entirely in its gaseous phase, the temperaturetolerance is even greater, and, in somce cases, temperature control maybe unnecessary.

This important aspect of the invention, that the operation of the deviceis much less sensitive to temperature variations than was the case withpreviously available devices, is well illustrated in FIG. 4, which is agraph showing the variation of permeation rate of sulfur dioxide (S0 innanograms (grams X 10') per minute (ng/min), plotted on a logarithmicscale, with temperature in degrees centigrade (C). The solid line 29 istypical of previous devices made for this purpose, while the broken line31 shows the improvement effected by the present invention, in which theS0 is held in liquefied form. For example, the permeation rate for bothdevices is approximately 315 ng/min at 262C. At 362C, the permeationrate of the old tube device increases to 730 ng/min, while the rate forthe new device increasesto only 410 ng/min; i.e., an increase of l32percent, or approximately 13 percent per degree centigrade, for the olddevice compared with an increase of 30 percent, or only 3 percent perdegree centigrade, for the new device of this invention. In thatparticular temperature range, then, the new device is approximately onlyone-fourth as sensitive to temperature changes as the prior device.

The variation of permeation rate with temperature can be considered ashaving two components: one due to a vapor pressure change resulting fromthe temperature change, and the other due to an inherent property of thepermeable material that results in a change in permeation rate when thetemperature is varied at constant pressure. The first component canbeeasily determined from vapor pressure tables. For example, for S0 thevapor pressure increases from 33.45 p.s.i. to 66.45 p.s.i. for atemperature change from 10C to 30C, i.e. the vapor pressure increasesalmost exactly by a factor of two. Thus, the permeation rate will bedoubled by an increase of temperature from l0C to 30C because of thevapor pressure increase.

This first component of the permeation rate variation is showngraphically by'the curve 32 in FIG. 4. Note that only the slopes of thelines 29, 31 and 32 are of interest, since the points of intersectionwith the y axis can be varied by changing the cross-sectional area ofthe permeable material.

Note also that the second component of the permeation rate variation,the component inherent to the permeable material, is positive for theprior device, resulting in the steeper characteristic curve 29, but isnegative for the new device of this invention, resulting in the improvedcharacteristic curve 31. Using the same S0 example, the permeation ratefor the prior device increases from ng/min at 10C to 440 ng/min at 30C.Of this 360 ng/min increase, only approximately 80 ng/min results fromthe vapor pressure increase (a twofold increase), and the remaining 280ng/min results from the unfavorable temperature characteristic of thepermeable material of the prior device. For the device of thisinvention, over the same temperature range, the permeation rateincreases from 200 ng/min to 350 ng/min, and a 200 ng/min increase dueto the vapor pressure change is offset by a 50 ng/min decrease due tothe different permeable material of the invention.

If the substance in the cylinder 10 is entirely in its gaseous phase,the variation of pressure in the cylinder will be nearly directlyproportional to the absolute temperature, in accordance with the idealgas laws. Thus, the permeation rate will also vary in direct proportionto the absolute temperature because of this pressure change, as shown bythe curve 33 in FIG. 5. A temperature change from 10C to 30C results ina first component of permeation rate increase of approximately 7percent, or from 200 ng/min to 214 ng/min in the curve 33 asillustrated.

As was seen in the foregoing discussion relating to the liquefied gasform of the invention, an increase in temperature from 10C to 30Cresults in a second component of permeation rate change of minus 50ng/min for the new device. Thus, the combined effect of the twocomponents is, as shown at 34 (FIG. 5), a net decrease of 36 ng/min,i.e., approximately 18 percent change in permeation rate for 20Ctemperature change, or less than 1 percent per C. With temperaturesensitivity this low, temperature control would be totally unnecessaryin some situations. Using only the gaseous phase of the substance in theprior devices does not, of course, greatlyimprove the temperaturecharacteristic for those devices, since there is still a large secondcomponent of permeation rate increase inherent to the material of thedevices.

Although the form of the invention using only the gaseous state of thesubstance suffers from a shorter life between refills and a gradualdecline in permeation rate, the device is still a practical one. Againusing S0 as an example, if the cylinder 10 is 8 or 9 inches long and 2or 3 inches in diameter, it will contain approximately 3 grams of S0 attwice atmospheric pressure. At a permeation rate of 1000 ng/min,thiswill last for more than 5 years, so that the permeation rate decrease isless than 2 percent per month, an entirely acceptable figure for mostapplications.

Another important aspect of the invention is that the passage 12 can beeasily dimensioned to select any of a wide range of permeation rates.Since the permeation rate is inversely proportional to the length of thepassage 12 and directly proportional to the cross-sectional area of thepassage, the rate may be varied by changing the length or diameter ofthe passage. In one form of the presently preferred embodiment, thepassage is accurately machined to be 0.016 inch in diameter and 0.25inch long. By way of example, a l00-fold increase in permeation rate canbe achieved by a IO-fold diameter increase, to 0.160 inch, with thelength kept constant. No increase in the size of the cylinder 10 wouldbe required unless it was needed to compensate for the shorter life ofthe device between refills, due to the faster permeation rate.

This availability of higher permeation rates allows the convenientproduction of accurate calibration samples with concentrations in thehundreds of parts per million, for use in the calibration of industrialstack monitors, for example, where accurate calibration was previouslycumbersome and difficult. For example, a high emission rate S tube ofthe old type would need to be about 50 inches long to generate 100 ppmof S0 at 22C in a carrier gas flowing at 1 liter per minute. Using thepresent invention, a passage diameter of only approximately 0.5 inchwould be needed, and the device need only be 2 inches in diameter and 8or 9 inches long to attain a life between refills comparable with theold device. Lower concentrations in the order of one part per millionare, of course, still obtainable from the device if an appropriatelysmaller diameter passage is employed.

The substance to be stored in and emitted from the cylinder 10 may beany of a wide range of fluids including, but not limited to, sulfurdioxide, nitrogen dioxide, hydrogen sulfide, ammonia, chlorine, bromine,hydrogen fluoride, propane, propylene, methyl mercaptan, ethyl oxide,n-butane, carbon tetrachloride, formaldehyde, acetaldehyde, and evenwater. The use of water as the gaseous substance has importantapplication in the accurate calibration of industrial hygrometers,widely used to monitor the environment for certain manufacturingprocesses. It will be understood that some of the substances named arenot suitable for storage entirely in the gaseous phase at ordinarytemperatures.

From the foregoing, it will be seen that the invention described hereinsatisfies a need for accurate, high concentration calibration samples,which were not hitherto conveniently available. Furthermore, theinvention is a significant improvement over prior devices of the sametype in that sensitivity to temperature variations is greatly decreased,and the device may be more easily connected to a fluid supply line.

Although one embodiment of the invention has been described in detailfor purposes of illustration, it will be appreciated that variousmodifications may be made without departing from the spirit and scope ofthe invention.

I claim:

1. A device for emitting a gaseous substance at a predetermined rate,comprising:

a sealed vessel for holding said substance under pressure, said vesselbeing impermeable to said substance and having an exit for emission ofsaid substance; and

a quantity of permeable polymeric silicone material positioned in saidexit, said material having a permeability which decreases withincreasing temperature and constant pressure in said vessel, wherebysaid substance permeates through said material and is emitted from saidvessel at the predetermined rate, and said device has acharacteristically low overall sensitivity to temperature variations.

2. A device as set forth in claim 1, wherein said quantity of permeablepolymeric silicone material is a dimethyl polysiloxane.

3. A device as set forth in claim 1, wherein said substance is held bysaid sealed vessel partially in the liquid phase.

5. A device for emitting a substance at a predeter- 5 mined rate,comprising:

a sealed vessel for holding said substance under pressure, said vesselbeing impermeable to said substance and having an exit passage foremission of said substance; and

a quantity of permeable polymeric plastic material completely blockingsaid passage, said material having a permeability which decreases withincreasing temperature and constant pressure in said vessel, wherebysaid substance permeates through said material blocking said passage andis emitted from said vessel at the predetermined rate, and said devicehas a characteristically low overall sensitivity to temperaturevariations.

6. A device as set forth in claim 5, wherein said permeable polymericplastic is a silicone polymer.

7. A device as set forth in claim 6, wherein said silicone polymer is adimethyl polysiloxane.

8. A device as set forth in claim 6, wherein said substance is held bysaid sealed vessel partially in the liquid phase.

9. A device as set forth in claim 5, wherein said permeable polymericplastic is a silicone polymer and said substance is held by said sealedvessel partially in the liquid phase, said device further includingtemperature control means to maintain said vessel and said substance ata substantially constant temperature.

10. A device as set forth in claim 9, further including removable vesselsealing means.

11. A device as set forth in claim 5, wherein said substance is selectedfrom the group consisting of sulfur dioxide, nitrogen dioxide, hydrogensulfide, ammonia, chlorine, bromine, hydrogen fluoride, propane,propylene, methyl mercaptan, ethyl oxide, n-butane, carbontetrachloride, formaldehyde, acetaldehyde, and water.

a quantity of a permeable silicone polymeric material completely fillingsaid passage and bonded to the walls thereof, said material having apermeability which decreases with increasing temperature and constantpressure in said vessel;

connection means for connecting said device to the moving fluid stream,whereby said substance permeates through said silicone material at thepredetermined rate and is emitted into the moving fluid stream throughsaid connection means, and said device has a characteristically lowoverall sensitivity to temperature variations.

13. A device as set forth in claim 12, further including sealing meansremovably attached to said vessel to allow filling and refilling thereofwith said substance.

14. A device as set forth in claim 13, wherein:

said vessel is a hollow cylinder; and

said passage is a tube through one end of said cylinder.

15. A device as set forth in claim 14, wherein said connection meansincludes a T-shaped pipe fitting connecting said cylinder to a lineenclosing the fluid stream.

16. A device as set forth in claim 12, wherein said substance isselected from the group consisting of sulfur dioxide, nitrogen dioxide,hydrogen sulfide,'ammonia, chlorine, bromine, hydrogen fluoride,propane, propylene, methyl mercaptan, ethyl oxide, n-butane, carbontetrachloride, formaldehyde, acetaldehyde, and water.

17. A device as set forth in claim 12, wherein said substance is held bysaid sealed vessel partially in the liquid phase in equilibrium with thegaseous phase.

18. A device as set forth in claim 12, wherein said substance is held bysaid sealed vessel solely in the gaseous phase.

19. A device as set forth in claim 12, further including temperaturecontrol means for maintaining said sealed vessel and said substance at asubstantially constant temperature, and thereby maintaining a constantrate of emission of said substance.

20. A device as set forth in claim 19, further including sealing meansremovably attached to said vessel to allow filling and refilling thereofwith said substance.

21. A device as set forth in claim 20, wherein:

said vessel is a hollow cylinder; and

said passage is a tube through one end of said cylinder.

22. For use with apparatus for the analysis of fluid mixtures, a devicefor generating a calibration mixture with a predetermined concentrationof a substance, said device comprising: 1

a vessel for holding said substance under pressure,

said vessel being impermeable to said substance and having a passagethrough which said substance is emitted into a moving stream of an inertfluid medium;

a body of a permeable silicone polymeric material completely fillingsaid passage and bonded to the walls thereof to limit emission of saidsubstance to a constant rate for a constant temperature, said materialhaving a permeability which decreases with increasing temperature andconstant pressure. in said vessel, thereby providing for said device acharacteristically low overall sensitivity to temperature variations.

23. A device as set forth in claim 22, wherein:

said vessel is a hollow cylinder; and

said passage is located at one end of said cylinder.

24. A device as set forth in claim 23, wherein said connection meansincludes a T-shaped pipe fitting connecting said cylinder to a pipeenclosing the moving stream of fluid.

25. A device as set forth in claim 22, wherein said substance isselected from the group consisting of sulfur dioxide, nitrogen dioxide,hydrogen sulfide, ammonia, chlorine, bromine, hydrogen fluoride,propane, propylene, methyl mercaptan, ethyl oxide, n-butane, carbontetrachloride, formaldehyde, acetaldehyde, and water.

26. A device as set forth in claim 22, wherein said substance is held bysaid sealed vessel partially in the liquid state in equilibrium with thegaseous phase.

27. A device as set forth in claim 22, wherein said substance is held bysaid sealed vessel solely in the gaseous phase.

28. A device as set forth in claim 22, further including temperaturecontrol means for maintaining said sealed vessel and said gaseoussubstance at a constant temperature.

29. A device as set forth in claim 1, wherein said substance is held bysaid sealed vessel entirely in the gaseous phase.

2. A device as set forth in claim 1, wherein said quantity of permeablepolymeric silicone material is a dimethyl polysiloxane.
 3. A device asset forth in claim 1, wherein said substance is held by said sealedvessel partially in the liquid phase.
 4. A device as set forth in claim3, wherein said permeable polymeric silicone material is a dimethylpolysiloxane.
 5. A device for emitting a substance at a predeterminedrate, comprising: a sealed vessel for holding said substance underpressure, said vessel being impermeable to said substance and having anexit passage for emission of said substance; and a quantity of permeablepolymeric plastic material completely blocking said passage, saidmaterial having a permeability which decreases with increasingtemperature and constant pressure in said vessel, whereby said substancepermeates through said material blocking said passage and is emittedfrom said vessel at the predetermined rate, and said device has acharacteristically low overall sensitivity to temperature variations. 6.A device as set forth in claim 5, wherein said permeable polymericplastic is a silicone polymer.
 7. A device as set forth in claim 6,wherein said silicone polymer is a dimethyl polysiloxane.
 8. A device asset forth in claim 6, wherein said substance is held by said sealedvessel partially in the liquid phase.
 9. A device as set forth in claim5, wherein said permeable polymeric plastic is a silicone polymer andsaid substance is held by said sealed vessel partially in the liquidphase, said device further including temperature control means tomaintain said vessel and said substance at a substantially constanttemperature.
 10. A device as set forth in claim 9, further includingremovable vessel sealing means.
 11. A device as set forth in claim 5,wherein said substance is selected from the group consisting of sulfurdioxide, nitrogen dioxide, hydrogen sulfide, ammonia, chlorine, bromine,hydrogen fluoride, propane, propylene, methyl mercaptan, ethyl oxide,n-butane, carbon tetrachloride, formaldehyde, acetaldehyde, and water.12. A device for emitting a substance at a predetermined rate into amoving fluid stream to produce a desired concentration of said substancein the stream, said device comprising: a sealed vessel for holding saidsubstance under pressure, said vessel being impermeable to saidsubstance and having a passage through which said substance is emittedinto the moving fluid stream; a quantity of a permeable siliconepolymeric material completely filling Said passage and bonded to thewalls thereof, said material having a permeability which decreases withincreasing temperature and constant pressure in said vessel; connectionmeans for connecting said device to the moving fluid stream, wherebysaid substance permeates through said silicone material at thepredetermined rate and is emitted into the moving fluid stream throughsaid connection means, and said device has a characteristically lowoverall sensitivity to temperature variations.
 13. A device as set forthin claim 12, further including sealing means removably attached to saidvessel to allow filling and refilling thereof with said substance.
 14. Adevice as set forth in claim 13, wherein: said vessel is a hollowcylinder; and said passage is a tube through one end of said cylinder.15. A device as set forth in claim 14, wherein said connection meansincludes a T-shaped pipe fitting connecting said cylinder to a lineenclosing the fluid stream.
 16. A device as set forth in claim 12,wherein said substance is selected from the group consisting of sulfurdioxide, nitrogen dioxide, hydrogen sulfide, ammonia, chlorine, bromine,hydrogen fluoride, propane, propylene, methyl mercaptan, ethyl oxide,n-butane, carbon tetrachloride, formaldehyde, acetaldehyde, and water.17. A device as set forth in claim 12, wherein said substance is held bysaid sealed vessel partially in the liquid phase in equilibrium with thegaseous phase.
 18. A device as set forth in claim 12, wherein saidsubstance is held by said sealed vessel solely in the gaseous phase. 19.A device as set forth in claim 12, further including temperature controlmeans for maintaining said sealed vessel and said substance at asubstantially constant temperature, and thereby maintaining a constantrate of emission of said substance.
 20. A device as set forth in claim19, further including sealing means removably attached to said vessel toallow filling and refilling thereof with said substance.
 21. A device asset forth in claim 20, wherein: said vessel is a hollow cylinder; andsaid passage is a tube through one end of said cylinder.
 22. For usewith apparatus for the analysis of fluid mixtures, a device forgenerating a calibration mixture with a predetermined concentration of asubstance, said device comprising: a vessel for holding said substanceunder pressure, said vessel being impermeable to said substance andhaving a passage through which said substance is emitted into a movingstream of an inert fluid medium; a body of a permeable siliconepolymeric material completely filling said passage and bonded to thewalls thereof to limit emission of said substance to a constant rate fora constant temperature, said material having a permeability whichdecreases with increasing temperature and constant pressure in saidvessel, thereby providing for said device a characteristically lowoverall sensitivity to temperature variations.
 23. A device as set forthin claim 22, wherein: said vessel is a hollow cylinder; and said passageis located at one end of said cylinder.
 24. A device as set forth inclaim 23, wherein said connection means includes a T-shaped pipe fittingconnecting said cylinder to a pipe enclosing the moving stream of fluid.25. A device as set forth in claim 22, wherein said substance isselected from the group consisting of sulfur dioxide, nitrogen dioxide,hydrogen sulfide, ammonia, chlorine, bromine, hydrogen fluoride,propane, propylene, methyl mercaptan, ethyl oxide, n-butane, carbontetrachloride, formaldehyde, acetaldehyde, and water.
 26. A device asset forth in claim 22, wherein said substance is held by said sealedvessel partially in the liquid state in equilibrium with the gaseousphase.
 27. A device as set forth in claim 22, wherein said substance isheld by said sealed vessel solely in the gaseous phase.
 28. A device asset forth in claim 22, further including tempeRature control means formaintaining said sealed vessel and said gaseous substance at a constanttemperature.
 29. A device as set forth in claim 1, wherein saidsubstance is held by said sealed vessel entirely in the gaseous phase.